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Studies at the Liquid-Air Interface

The liquid-air interface comprises many natural systems including atmospheric processes such as heterogeneous cloud formation and ozone depletion. Our group has recently characterized the following systems:

  • Molecular trimethylamine (TriMA) on the surface of water
  • Ionic salt: tetramethylammonium hydroxide (TMAH)
  • Molecular ammonia (NH3) on the surface of water

The thermodynamic properties of water mixtures are also widely studied because equilibria between aqueous- and gas-phase species at liquid surfaces are relevant to industrial applications and atmospheric systems. Indeed, the predominant components of atmospheric aerosols and droplets are sulfates, nitrates, ammonia, amines and water, all of which are in chemical equilibrium with the surrounding air. Mass transfer of species into aerosol droplets has been investigated to better understand the role that ammonia, HCl, and inorganic acids play in atmospheric, heterogeneous processes. Uptake of ammonia, for example, is believed to occur by molecular diffusion towards a droplet surface, followed by penetration of the air/water interface, hydrolysis, and diffusion into the bulk liquid. Recently, it has also suggested that the organic compounds such as amines might play an essential role in aerosol formation and growth. Typical experiments deduce the mass accommodation coefficient, of species from the gas phase into water droplets to describe processes occurring on atmospheric aerosols. An important missing component of the accommodation picture is determination of the surface structure of mixtures characteristic of atmospheric droplets. Surface structure information is invaluable for understanding aerosol neutralization pathways and aerosol growth processes. Because of the surface specificity of SFG, it is ideally suited for investigating the structure of such liquid surfaces.

Heterogeneous Stratospheric Chemistry

H2SO4/H2O solutions are key players in stratospheric ozone loss. It is now widely accepted that sulfate aerosols participate in ozone depletion by: (1) serving as surfaces for chlorine activation, (2) affecting the NOx budget, and (3) being the starting material for Polar Stratospheric Clouds (PSCs). Although some field observations and laboratory work corroborate this general picture, a fundamental understanding of these heterogeneous processes is lacking. The primary goal of this research is to study surfaces of H2SO4/H2O solutions in contact with atmospherically relevant gases, e.g. HCl, HNO3, and HOCl, to provide molecular details of heterogeneous reactions. Incorporating our results into atmospheric models will lead to a fuller and more accurate picture of ozone depletion. This information will aid researchers, and thereby policy makers, in making vital decisions about issues such as deploying a fleet of civilian Super-Sonic-Transports (SSTs) and choosing CFC substitutes.

Recently, we have used SFG to study surfaces of H2SO4/H2O solutions at various mole fractions, including the relevant 0.1-0.4x, characteristic of stratospheric sulfate aerosols at mid-latitudes. At 0.1x H2SO4, water molecules are fairly ordered on the surface; at 0.4x, the surface resembles pure H2SO4. These findings indicate that reactions occurring at the surface of stratospheric sulfate aerosols may be inhibited at the higher sulfuric acid concentrations. Experiments with ionic sulfate solutions were performed to further investigate how sulfates orient surface water molecules. Results from both studies indicate that negative and positive ions set up a double layer and reorient water molecules. Water orientation and concentration greatly depends on the ability of solutions to form complexes.

Now that we have characterized the surface of sulfuric acid solutions, we are in the process of introducing HCl into the H2SO4/H2O system. The goal of this research is to identify chemical species responsible for heterogeneous activation of odd chlorine at mid-latitudes, and to determine how effectively these sequester odd chlorine. It is generally believed that sulfate aerosol surfaces serve to convert relatively stable reservoir species, e.g. HCl and ClONO2, into photolabile species like Cl2 and HOCl, e.g.:

HCl + ClONO2----> Cl2 + HNO3 (1)

Photolysis of Cl2 then directly destroys ozone in catalytic cycles. Concentration and orientation of both surface HCl and HOCl molecules are being characterized to determine if heterogeneous reactions occur on the surface of aerosols at the gas-liquid interface. These results will help develop a reaction mechanism that indicates if these molecules adsorb onto the surface, or diffuse into the bulk. For more information about these atmospheric processes, see: 1,2


Spectrum of Water

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